1. Field Of The Invention
This invention relates to continuously variable transmissions (CVTs) and the noise developed as a result of the use of a power transmission chain-belt to transfer torque between the transmission input and output sheaves. More particularly, this invention relates to a control system for varying the drive sheave ratio in a CVT at a rate differing from the belt-pass frequency in order to provide a lower noise chain-belt drive system.
2. Description Of The Prior Art
Continuously variable transmissions that include a variable pulley arrangement for transferring torque from an input shaft to an output shaft have been known for some time. In such transmissions, a first variable pulley is mounted on a first, or input, shaft and a second variable pulley is mounted on a second, or output shaft. A flexible belt of metal or elastomeric material interconnects the two pulleys to transfer torque between them. The primary or first pulley is driven by the engine and the output of the secondary or second pulley drives the drive train of the vehicle. Each pulley has one sheave axially movable relative to the other sheave to change the effective pulley diameter. By varying the effective pulley diameter of the first pulley, the transmission belt can be forced outward on the first pulley and pulled inward on the second pulley or vice versa. The movement of the sheave of the primary pulley servo regulates the transmission ratio across the CVT. The hydraulic pressure of the sheave of the secondary pulley servo varies the clamping force on the chain-belt connecting the primary and secondary pulleys. Maximum operating efficiency of the engine can be approached by varying the ratio across the pulleys to adjust the transmission to give an optimum speed ratio for each specific load.
Flexible continuous rubber belts have been employed to transfer torque between the first and second pulleys of a CVT. These belts, however, are subject to wear as a result of torque loads and adverse conditions. For CVT usage, flexible metal chain-belts have therefore been developed.
Flexible belts for use with a CVT are generally of two types, those referred to as "push" belts and those referred to as "pull" belts. An examples of a "push" belt is described in Van Doorne et al. U.S. Pat. No. 3,720,113 and an example of a "pull" belt can be found in Cole Jr. et al. U.S. Pat. No. 4,313,730. The push type belts include an endless array of nested metal bands and an endless array of load blocks free to move longitudinally along the metal bands.
The pull type belt typically includes an endless chain as the carrier made from a plurality of links pivoted by pivot pins with a plurality of load blocks encircling the links. The pivot pins prevent the load blocks from moving longitudinally relative to the links. The load blocks are generally trapezoidal in shape and have edge surfaces for engaging the flanges of the pulleys for transferring torque between the pulleys. Pull type belts tend to be simpler and therefore less expensive to manufacture than the push type belts as the chain links and load blocks can be of composite width allowing the links and load blocks to be stamped from sheet material. In addition, the pins can be cut from extruded stock also keeping costs low. The present invention relates to CVTs with both push-type and pull-type belts.
The use of articulated chain-belts to transfer torque from the input shaft to the output shaft of a CVT results in a noise spectrum generated by the engagement of the load blocks and the pulley flanges. The generated noise spectrum generally has peaks providing discernable pure tones. A noise spectrum which has peaks at certain frequencies in a regular pattern is less acceptable to the human ear than white noise, that is, a noise spectrum with an irregular pattern, or lack of pure tones. A white, irregular or arrhythmic noise is less noticeable and annoying than a noise spectrum which contains recognizable pure tones of a single frequency.
One way in which the generation of these pure tones has been understood is in terms of forces acting to squeeze the input and output shafts of the CVT together. These forces are transferred to the shaft bearings and into the casing of the CVT where vibrations are set up that generate pure tones. The forces on the pulley shafts are believed to be produced as a result of the articulation of the chain-belt.
An articulated chain-belt does not conform exactly to the shape of the pulleys as it follows its path about the sheave. Instead, it forms a polygon that closely resembles the curvature of the pulleys. This relationship results in a condition that is described as chordal action. As the belt moves around the pulleys, the motion of the chain-belt includes a radial as well as a longitudinal component. This radial motion changes the effective length of the chain-belt and creates a cyclic tensioning in the portion of the chain-belt connecting the two pulleys.
For belts having a regular pitch length, known as "straight pitch" belts, the tensioning cycle will occur at the rate at which successive pitches of the chain pass a given point. This frequency is termed the "pitch-pass frequency" or "belt-pass frequency". As a result of the cyclic tensioning, the input and output shafts are squeezed together and released at the pitch-pass frequency. This relative motion of the input and output shafts is small but results in vibrations being transmitted to the casing of the transmission system, which although small in amplitude are of relatively high energy. These vibrations generate a noise pattern that includes a discernable peak at the pitch-pass frequency which is perceived at an unacceptably high level by the human ear.
Several chain-belts of different constructions have been developed to modify the pitch-pass frequency in order to reduce the amplitude of the noise peaks at this frequency and make the perceived level of noise generated more acceptable. These belts are termed "random pitch" belts. For example, U.S. Pat. No. 4,650,445 describes a chain linked at pivot points by a pair of pivot pins designed to vary the chordal action of at least some of the links to alter the pitch-pass frequency and thereby modify the noise patterns generated. U.S. Pat. No. 4,516,964, describes load blocks of different transverse widths, all of which contact the pulley flanges but some at different radial locations of the sheaves. The regularity of the pitch-pass frequency is therefore modified and the noise pattern modified. U.S. Pat. No. 4,516,965, describes load blocks of different transverse widths that do not contact the pulley flanges to modify the generated noise pattern. Alternatively, load blocks are "skipped" to modify the generated noise pattern. U.S. Pat. No. 4,516,963 describes load blocks having a random mixture of load block pulley flange engaging areas to provide random engagement and thereby modify the noise patterns generated.
The prior art chain-belts described in U.S. Pat. Nos 4,516,963, 4,516,964, 4,516,965 and 4,650,445, incorporated herein by reference and discussed above, are effective in modifying the contact pattern to reduce the amplitude of noise at the pitch-pass frequency. The present invention is directed to modification of the chain-belt contact pattern by a variation in the sheave ratio at a rate different from the belt-pass frequency.